(1) Field of the Invention
This invention relates to a pressure-resistant paper vessel. More particularly, it relates to a pressure-resistant vessel excellent in the adaptability to thermal disposal, which is suitable for containing and storing therein a carbon dioxide gas-containing drink such as beer or carbonated drink and in which the barrel portion and lid members are composed mainly of paper.
(2) Description of the Prior Art
A vessel or container for beer or carbonated drink is different from a vessel for milk or refreshing drink such as juice in the point where the vessel should sufficiently resist the inner pressure produced by carbon dioxide gas and should prevent permeation and escape of carbon dioxide gas. Accordingly, a glass bottle or aluminum can has heretofore been used as the vessel for beer or carbonated drink. From the resource-saving viewpoint, the glass bottle is preferred because it can be collected and used repeatedly. However, since the thickness of glass constituting the bottle is large, the glass bottle is heavy in weight and bulky in size and hence, the transportation cost is large. Moreover, there is a risk of an accident due to explosion at a high temperature in summer or by mishandling. Furthermore, since the glass bottle is used repeatedly, various steps are necessary for washing an empty bottle, checking a washed bottle and treating a washing liquid. Thus, the glass bottle is not satisfactory from the economical viewpoint. Since the mechanical strengths of the metal vessel is much higher than those of the glass vessel, the thickness and weight of the metal vessel can be reduced, and the metal vessel is sufficient in the physical properties such as pressure resistance and gas barrier property. However, there recently arises a problem of disposal of empty cans as a social problem, and the metal vessel as a throwaway vessel should now be reconsidered.
A polyester bottle (PET bottle) formed by blow molding has recently been marketed. This polyester bottle has attracted attention in the art because it has a light weight, is thermally disposable and is excellent in the design property. However, the polyester bottle causes pollution of environment with used bottles as in case of the metal vessel, and it cannot easily been burnt up at an ordinary incineration plant.
Although these conventional vessels such as glass bottles, metal cans and PET bottles involve difficult problems, since there has not been developed a new pressure-resistant vessel capable of solving these problems, these conventional vessels are used even at the present in spite of these problems. Under this background, instead of these conventional pressure-resistant vessels for carbon dioxide gas-containing liquid drinks such as beer and carbonated drink, a vessel composed of paper is eagerly desired because paper is abundant, light in weight and cheap, thermal disposal of the used paper vessel is very easy and the used vessel can be used repeatedly as paper stock. However, paper has various problems. For example, the physical properties become deteriorated on absorption of the moisture, the gas permeability is very high, and it is very difficult to form paper into a shape having a three-dimensional curved surface, though this is relatively easy in case of glass, metals and plastics. Because of these defects, development of pressure-resistant paper vessels have been hindered.
As means for imparting a water-proof property to paper, there have ordinarily been adopted a wax-coating method and a polyethylene film-laminating method. Most of milk vessels of paper now marketed in large quantities are manufactured by using polyethylene film-laminated paper.
In case of vessels for liquids such as sake, wine and soy source, not only the water-proof property but also the gas barrier property is required, and only the water-proof treatment is insufficient but the gas barrier property should further be given to paper. As means for imparting a gas barrier property, an aluminum foil is ordinarily used, and a vessel composed of aluminum foil-laminated paper is marketed in considerable quantities as the vessel for sake or the like. A wall of this paper vessel ordinarily has a very complicated structure of polyethylene/paper/polyethylene/adhesive/aluminum foil/adhesive/polyethylene.
Conventional paper vessels for milk, sake, fruit juice and the like are box-shaped vessels in which each of the barrel portion, bottom face portion and the top face portion is flat. If beer or carbonated drink is filled and stored in such a box-shaped vessel, the flat portion is liable to be deformed into a bulgy shape by the inner pressure and a large deformation is caused on the ridgeline of the bent part, resulting in breakage or leakage of gas and destruction of the functions of the vessel. The pressure resistance required for a pressure-resistant vessel for a carbon dioxide gas-containing liquid such as beer or carbonated drink is considerably high. For example, beer has an inner pressure of 2 to 2.5 kg/cm.sup.2 G at normal temperature or 4 to 5 kg/cm.sup.2 G at 50.degree. C., and carbonated drink has an inner pressure of 3 to 3.5 kg/cm.sup.2 G at normal temperature or 5 to 6 kg/cm.sup.2 G at 50.degree. C. Accordingly, in order to impart a sufficient pressure resistance to the vessel, it is necessary that the majority of the portion falling in contact with a compressed fluid should be formed to have a curved surface so that the internal stress is uniformalized over the entire wall of the vessel. In fact, all of the conventional pressure-resistant vessels such as glass bottles, metal cans and PET bottles are formed to have such a shape as satisfying this requirement. Also in a paper vessel, this point should be sufficiently taken into consideration. Namely, from the viewpoint of the mechanical strengths, particularly, the pressure resistance, it is preferred that the barrel portion should have a cylindrical shape and the lid portion should have a spherical curved shape.
In order to obtain a satisfactory pressure-resistant vessel, the following three problems should simultaneously be solved: namely, how to impart a water-proof property, how to prevent permeation of gas and how to impart a strength sufficient to resist the inner pressure. However, it is very difficult to simultaneously solve these problems, and therefore, a pressure-resistant vessel composed entirely of paper has not been marketed.
As pointed out hereinbefore, as means for imparting a water-proof property and a gas barrier property to paper, there is ordinarily adopted a method in which an aluminum foil considered to have a highest reliability as the gas barrier layer is laminated on paper. The permeation speed of gases such as oxygen, water vapor and carbon dioxide gas through the aluminum foil is substantially zero and the aluminum foil is excellent as the gas barrier material over conventional gas barrier plastic films. Namely, a most excellent gas barrier plastic film now marketed has an oxygen permeability 2 to 50 cc-mil/m.sup.2 .multidot.24 hours.multidot.atm and a carbon dioxide gas permeability of 5 to 200 cc-mil/m.sup.2 .multidot.24 hours.multidot.atm at normal temperature.
It has been found that serious problems as described below arise when an aluminum foil is applied to a pressure-resistant paper vessel. In the first place, as pointed out hereinbefore, in order for a pressure-resistant vessel to resist the inner pressure, it is preferred that the majority of portions falling in contact with a compressed fluid should have a curved surface, for example, a three-dimensional curved surface. However, it is difficlt to laminate an aluminum foil to a lid member having such a curved surface. Furthermore, an additional step of shaping the aluminum foil in advance according to the shape of the lid member is necessary. In the second place, in an aluminum foil-laminated paper vessel, the aluminum foil cannot follow a slight deformation caused by the inner pressure and breakage of the aluminum foil takes place, with the result that leakage of gas is often caused at the stage of small deformation before appearance of breakage due to a large deformation of the paper layer. In the third place, although the aluminum foil has an excellent gas barrier property, if the thickness of the aluminum foil is as small as 7 to 10 .mu.m, pinholes are formed and such a thin aluminum foil cannot be an excellent gas barrier material. It is said that according to the present technique, it is impossible to reduce the thickness of the aluminum foil below 30 .mu.m without formation of pinholes. Incidentally, use of an aluminum foil having such a large thickness results in increase of the price of the vessel. Accordingly, at the present, an aluminum foil having a thickness of 5 to 10 .mu.m is used with a risk of formation of pinholes, and from the practical viewpoint, the reliability of this aluminum foil concerning the gas barrier property cannot be regarded as being complete.
As is apparent from the foregoing description, an aluminum foil is insufficient in various points as a gas barrier material for a pressure-resistant paper vessel. Accordingly, development of an excellent gas barrier material capable of being used as a substitute for the aluminum foil is eagerly desired. This gas barrier material should have a tensile elongation higher than that of an aluminum foil, should be excellent in the processability, should not cause environmental pollution and should easily be laminated on paper.
A gas barrier plastic film is considerably inferior to the aluminum foil in the gas barrier property, and therefore, the thickness of the film should considerably be increased, resulting in increase of the price of the vessel.
An example of a multi-ply packaging material comprising a gas barrier plastic film is disclosed in Japanese Patent Publication No. 56-40032 (1981). In this patent reference, it is taught that in a multi-ply packaging material comprising a plurality of gas barrier plastic film layers, if these layers are bonded together only through the peripheral edge portions thereof, the gas barrier characteristics are unexpectedly increased over the gas barrier characteristics of a single-ply layer of the same material having a thickness corresponding to the total thickness of the two film layers. From the data shown in working examples of this patent reference, it is seen that the oxygen or carbon dioxide gas permeability of a two-ply packaging material is reduced to 1/2 to 1/3 of the oxygen or carbon dioxide gas permeability of a single-ply packaging material having a thickness corresponding to the total thickness of the respective layers of the two-ply packaging material. The reasons for this phenomenon given in the Japanese reference are as follows. Namely, in the first place, in a multi-ply material having a certain total thickness of the respective layers, the time required for attaining a stable state of the permeation of gas is much longer than the time required for attaining this stable state in a single-ply material having a thickness corresponding to said total thickness. In the second place, this stable state of the permeation of gas is more readily disturbed by the adoption of the multi-ply structure. Namely, the pressure of gas which has passed through the first layer is reduced, and this reduction of the pressure results in substantial reduction of the capacity of permeating through the second or subsequent layer.
Ordinarily, it is understood that when a plastic film is exposed to a certain gas, the gas is first dissolved in the plastic film and the gas permeability is gradually increased from an initial small value to a substantially fixed value attained when the gas is dissolved in the film to the saturation point. The gas permeability at this stable equilibrium state is ordinarily accepted as the gas permeability of the plastic film.
We noted the above-mentioned phenomenon shown in Japanese Patent Publication No. 56-40032, especially the change of the permeability with the lapse of time during the period required for attaining the stable state of the permeation of gas (hereinafter referred to as "transition period"), and we made various experiments by using carbon dioxide gas and arrived at the following conclusions.
(1) In case of the multi-ply structure disclosed in the above-mentioned patent reference, the time required for attaining the stable state of the permeation of gas is much longer than in case of a single-ply structure of the same material having the same thickness. This time varies depending upon the kind and thickness of the gas barrier material and the measurement conditions (temperature and humidity).
(2) At the initial stage of the transition period, an apparently considerably low value is obtained by the measurement, and, when the comparison is made based on customarily mentioned gas permeabilities, the measurement should be performed after the stable state has sufficiently been attained.
(3) In the above-mentioned patent reference, it is taught that the gas permeability after attainment of the stable state in case of the two-ply structure is reduced to 1/2 to 1/3 of the gas permeability of the single-ply structure having the same thickness. However, from the results of the experiments made by us, it was confirmed that the gas permeability of the two-ply structure after attainment of the stable state is reduced only to about 80 to about 90% of the single-ply structure. It is construed that in the above patent reference, the conclusion was hasty drawn without the time required for attaining the stable permeation state being sufficiently taken into consideration.
In case of a vessel for beer or carbonated drink, it is said that it is sufficient if the quality-guaranteeing period or required shelf life is about 6 months. Accordingly, even when the gas permeability after attainment of the stable permeation state is high, if the time required for attaining the stable permeation state, that is, the transition period, is long, the material is very significant from the practical viewpoint. The multi-ply structure disclosed in Japanese Patent Publication No. 56-40032 is practically significant in the point where this transition period is prolonged. However, it was found that this transition period of the multi-ply structure disclosed in the above patent reference is still insufficient in view of the fact that the shelf life of a vessel for beer or carbonated drink is regarded as 6 months, and therefore, this multi-ply structure is not practically adopted for a vessel for beer or carbonated drink. Moreover, it was found that the gas permeability of the multi-ply structure after attainment of the stable permeation state is reduced only to 80 to 90% of the gas permeability of the single-ply structure and because of the low carbon dioxide gas barrier property, it is not permissible to apply the above-mentioned multi-ply structure to construction of a vessel for beer or carbonated drink.
Accordingly, we made research with a view to developing a layer structure having a much prolonged transition period and having a much improved gas barrier property after attainment of the stable gas permeation state. We first examined influences of a space between adjacent films in a multi-ply structure, and as the result, it was found that the presence of a very thin air layer greatly prolongs the transition period and if the thickness of this air layer is increased, the transition period is further increased and a much longer time is necessary for attaining the stable permeation state. In the multi-ply structure disclosed in the above-mentioned patent reference, the peripheral edge portions of the films are bonded together and in other portions, the films are merely piled and contacted with each other, and the thickness or function of the air layer present between the films is not mentioned at all.
The reasons why increase of the air or space layer between gas barrier films results in prolongation of the transition period are considered to be as follows. A gas which has passed through a first film is diluted in a space layer as described above and the ability of the gas to permeate through a subsequent film is reduced, and therefore, the gas permeability is controlled to a low level until the concentration of the gas in the space layer is increased to an equilibrium level. Accordingly, the larger is this space layer, the longer becomes the transition period. However, it was found that if only the space layer is increased in volume, though the transition period is prolonged, the level of the gas barrier property after attainment of the stable gas permeation state is substantially equal to that of the above-mentioned multi-ply structure.